Project Summary Genomic instability is a hallmark of cancer. On one hand, genomic instability of cancer cells promotes loss of tumor suppressors and activation of oncogenes. On the other hand, genomic instability renders cancer cells susceptible to radiation and chemotherapy. Recently, the concept of ?synthetic lethality? has been successfully used to exploit the genomic instability of cancer cells. For example, the DNA repair-defective BRCA1/2-deficient cells are highly sensitive to PARP inhibitors. In this application, I propose a new synthetic lethal strategy to target cancer cells expressing APOBEC3A with inhibitors of the ATR checkpoint kinase. My proposed studies may reveal a key function of ATR in response to APOBEC3A-induced DNA damage. Developing a strategy to specifically kill APOBEC3A-expressing cancer cells will be a breakthrough in targeted cancer therapy. My career goal is to obtain a research faculty position at a leading institute where I will dissect the mechanisms of APOBEC3A-causing genomic instability in cells. However, my successful transition to independence in this field would be significantly bolstered by augmenting my expertise in cell biology techniques with new training opportunities in mouse cancer models. It is with these acquired skills that I will be able to investigate whether ATR inhibitors can be used to specifically target cancer cells expressing APOBEC3A. The success of this project will be greatly enhanced by the outstanding collaborators that I have assembled to advise me throughout my transition to independence. In addition, the exceptional research environment available at MGH and the Harvard Medical School area has all of the necessary resources required for the proposed training and research studies. The K99/R00 award would provide me with the protected time needed for this advanced training and allow me to continue to foster my growth under the mentorship of Dr. Zou. I expect that the protected time provided by this award will allow me to elucidate the function of ATR in cells overexpressing APOCBEC3A and to determine whether ATR inhibitors (ATRi) can be used to promote synthetic lethality in cancer cells with high levels of APOBEC3A. In aim 1, I will elucidate how ATR inhibition leads to cell death in APOBEC3A expressing cells. In aim 2, I will determine what is the target of APOBEC3A in human cells. Finally in aim 3, I will investigate whether ATR inhibitors can be used to specifically target cancer cells expressing APOBEC3A. Despite my recent training in biochemistry and cell biology, I will need 1-2 additional years of training to establish myself as an independent investigator. Receipt of this award would not only allow me to expand my research plan, but also establish myself as a primary investigator in the field of cancer biology